From the VMware Workstation 9.0 Release Notes:
Hyper-V
Hyper-V has been added to the Workstation 9 guest operating system list. This enables customers to run Windows 8 with Hyper-V enabled, or install Hyper-V Server. This can be used for educational purposes or for building prototype Hybrid Clouds. This feature is NOT SUPPORTED and probably never will be. Microsoft does not support nesting of their hypervisor which makes it extremely difficult - if not impossible for VMware to fix issues that may occur in this configuration. For this reason, this capability has been implemented purely to see if we could do it!
CAUTION: DO NOT ATTEMPT TO RUN HYPER-V ON A VIRTUAL MACHINE IN PRODUCTION.